Conventionally, an image forming apparatus including a laser printer, which has such a structure that an endless belt is employed for conveying sheets of paper and for conducting intermediate transfer, has been known. In the image forming apparatus having this structure, there is generally provided a cleaning unit including a roller and/or a brush, in order to remove foreign substances such as toner, paper dust, etc., which have adhered to a surface of the belt. In an image forming apparatus disclosed in JP-A-9-152788, for example, a cleaning roller that is in pressure contact with the surface of the belt is driven to rotate so that foreign substances, which have been shifted from the belt to the cleaning roller, is scraped off by a blade. The cleaning unit of this type is usually driven, making use of power of a motor for driving the belt.